This invention relates to injection molding and more particularly to an improved hot runner system with a central integral heated probe.
In the past, it has been known to provide injection molding systems with elongated probes with internal heaters which the melt flows around on its way to the gate. One of these is shown in the applicant's previous U.S. Pat. No. 4,376,244 entitled "Injection Molding Heated Probe" which issued Mar. 8, 1983 which discloses a probe with a somewhat similar integral construction. More recently, the applicant found it is advantageous to provide externally open melt channels on the surface of the probe and the structure is shown in the applicant's U.S. patent application Ser. No. 453,572 entitled "Method of Manufacture of an Injection Molding Integral Heated Probe" which was filed Dec. 27, 1982. However, these prior systems have the disadvantage that, during operation, the insulative space that is provided between the hot probe and the surrounding cool melt necessarily fills with melt. It is, of course, well known to use melt as an insulator and this arrangement works well for some applications and some materials. The problem is that the melt in this space around the probe solidifies adjacent the cool outer wall, but does not entirely solidify adjacent the heated probe. Thus, when the material or even the colour of the material being molded changes, the previous material remains trapped in this space and introduces contamination into the molded product for a considerable period of time. Furthermore, with the trend to molding more difficult and less stable materials, it has been found that materials such as some heat sensitive flame retardant materials cannot satisfactorily be molded with these previous systems because of degradation of the stagnant material which is trapped in this space around the probe.
As may be seen in FIGS. 4 and 5 of Mold-Masters Limited brochure 3/82 printed in 1982, the applicant has previously attempted to overcome these problems in systems in which the melt passage extends centrally through a heated nozzle by filling the space around the nozzle with TEFLON (Trade Mark of DuPont for polytetrafluoroethylene insulation). While the use of a central heated probe which the melt flows around, rather than a heated nozzle with a central bore provides advantages in many instances, it has not previously been thought possible to overcome these problems because the melt flow occurs through this same space where the problems arise. Earlier solutions to these problems for systems having a center runner are shown in the applicant's U.S. Pat. Nos. 4,043,740 which issued Aug. 23, 1977 and 4,279,588 which issued July 21, 1981 which disclose seals bridging the gap around the gate to prevent the space around the nozzle filling with melt.
Another known practice for dealing with this problem is to first shoot a different material with a high melting temperature such as NYLON 66 through the system. This does fill the dead spots and avoid stagnation, but it has the disadvantage that it takes time to do and the whole system has to be cleaned if it is not done right or has to be removed later.